An annular combustion chamber of a turbomachine is defined by two walls in the form of bodies of revolution that are coaxial with one extending inside the other and that are connected together at their upstream ends by an annular end wall of the chamber.
The chamber end wall includes orifices for mounting means for injecting a mixture of air and fuel into the combustion chamber, the air coming from a compressor of the turbomachine and the fuel being delivered by injectors. The injectors extend substantially radially from their outer ends that are fastened on an outer casing of the combustion chamber, and they have heads that are aligned axially with the orifices in the chamber end wall.
The outer wall of the chamber includes at least one orifice for passing one end of an ignition spark plug, with the other end of the spark plug being fastened to an outer casing of the combustion chamber, said spark plug being used for initiating combustion of the air and fuel mixture inside the chamber.
During operation of the turbomachine, the walls of the combustion chamber expand thermally, thereby causing relative movement to take place between the outer wall of the combustion chamber and the ignition spark plug, and also between the chamber end wall and the fuel injectors.
In order to compensate for and allow these relative movements, guide devices are used for the spark plug and the injectors, each guide device comprising a ring and a bushing that are substantially coaxial, one being mounted inside the other, the ring being designed to have the spark plug or the injector passing axially therethrough and including an outer annular rim that is guided transversely in an inner annular groove in the bushing that is itself fastened to the edge of the orifice in the outer wall of the chamber or carried by the injection system. The bushing comprises two coaxial annular parts that are fastened to each other by welding and that define between them an annular groove for guiding the rim of the ring. European patent application EP-A1-1 770 332 in the name of the Applicant describes a guide device of that type.
The relative movements between the outer wall of the combustion chamber and the spark plug take place mainly in a direction parallel to the longitudinal axis of the combustion chamber. In operation, the bushing secured to the wall of the combustion chamber is thus moved essentially in said longitudinal direction relative to the ring for guiding the spark plug that is carried by the outer casing. The relative movements of the ring in the groove of the bushing in other transverse directions are of smaller amplitude. The axial or longitudinal direction is thus the privileged direction for movement of the ring for guiding the ignition spark plug relative to the bushing carried by the chamber.
Relative movements between the chamber end wall and the injectors take place mainly in directions that are radial relative to the longitudinal axis of the chamber, so the privileged direction for relative movement of the ring mounted around an injector is therefore a direction that is radial relative to the longitudinal axis of the combustion chamber.
In the prior art, the two annular parts forming the bushing of a guide device are fastened to each other at their outer peripheries by one or more welding beads extending around the entire outline of the bushing. Although the operator performing the welding operation takes great care in carrying out the welding, it is possible for the welding beads to spill into the annular groove for guiding the ring, thereby locally obstructing the outer periphery of the groove and thus reducing or preventing the ring from moving in a transverse direction in the groove of the bushing. The operation of welding the parts of the bushing is thus difficult to perform and does not enable welding spillage to be controlled. Furthermore, there is a significant risk of the device being blocked or jamming each time the rim of the ring comes into contact with a welding bead projecting into the groove of the bushing, with an associated unacceptable risk of the spark plug or the injector being broken.
One solution to that problem would be to overdimension the parts of the bushing so that the annular groove presents a transverse dimension that is greater than necessary in order to conserve sufficient clearance for movement of the ring in a transverse direction even when welding beads project into the inside of the groove.
Nevertheless, that solution is unsatisfactory since it leads to a significant increase in the weight of the device and it would impede ventilation of the wall of the combustion chamber by partially overlapping air-passing orifices in said wall situated in the vicinity of the device.